Booking a trip is always a bit of a gamble. No matter how carefully you time your purchase or comparison shop, travel prices fluctuate so often and wildly that you can never be sure they won't drop later (meaning, you overpaid). Now the good news: Whether you've booked a flight, made a hotel reservation, or rented a car, most allow you to take advantage of price fluctuations after you buy—as long as you know about them. Here are the free tools you can use to get the lowest travel prices—effortlessly and automatically.

AutoSlash: Automatic Price Reductions on Car Rentals

Book your car rental at AutoSlash and the site will search for and apply any coupon codes it can find for you. Then, up until your pickup date, the free service continues to track multiple vendors' rental rates and discounts several times a day and re-books for you if it finds a better deal. According to a recent NY Times article, the service claims to successfully lower car rental prices 85 percent of the time, with roughly half of those price cuts due to applying discount codes (that you don't have to search for or dig up yourself). The average savings if you book at least a couple of weeks ahead? A not-too-shabby 25 percent. You can cancel your reservation at any time.

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Unfortunately, not all car rental companies are keen on this idea. Both the Enterprise, National, and Alamo franchise and the Avis and Budget one refuse to let AutoSlash inventory their car rentals. Hertz and sister companies Dollar and Thrifty used to work with AutoSlash but recently pulled out. That leaves mostly the smaller/budget car rental chains, such as Fox Rent a Car, Payless, and a few others and makes the car rental search results more sparse. But if you don't care which car rental company you patronize and just want the best deal you can get without having to search for it yourself, give AutoSlash a try to see what's available where you're traveling.

Tingo: Automatic Refunds on Hotel Reservations

Hotel booking site Tingo works similarly to AutoSlash. After booking a hotel room at the site, the service continues to monitor the hotel's rates. If the rates drop after you book, Tingo automatically re-books your reservation and automatically refunds your credit card the difference, with no need for you to actually fill in any forms or such. You can set the price drop threshold (e.g., re-book only if the savings are over $10), as well as cancel your reservation at any time.

Not all hotel rooms are eligible for the auto-re-booking and automatic refunds, because some hotels set their room rates as non-refundable (unlike most car rentals, where you can cancel at any time). When you book, you'll need to accept the hotel's own cancellation fees, if there are any.

If you don't want to book on Tingo, you can still use the service to just track hotel prices and be notified when a hotel's price drops. In the last month, according to the service, 476 hotels in Paris had price drops, 273 in London, and 214 in NYC—lots of savings travelers might have missed out on if they weren't tracking price drops.

Yapta: Refund Alerts and Claim Assistance for Airline Tickets

Unlike AutoSlash and Tingo, previously featured airline ticket price tracker Yapta doesn't automatically re-book your flight. That's because once you buy a plane ticket, you typically face cancellation fees and penalties for any changes. Yapta, however, offers a kind of price drop protection that makes sure you get the best deal on your flight even after you purchase it. The service monitors your flight's price and, if it drops, notifies you so you can claim your airline refund and helps you submit your claim.

Alaska Airline and JetBlue offer refunds for any drop in price, while other airlines issue refunds depending on how much the price drops (for Virgin, it's $75 or more; United, American Airlines, Delta, and US Air, $150 or more). About 13 percent of Yapta-tracked fares were eligible for refunds in the first quarter of last year according to this MSN Money article. Since the service launched 5 years ago, average annual savings per Yapta member is $334.16.

(Orbitz offers a lowest price guarantee for flights and hotels with automatic refunds, but it's harder to ever get that, because the refund depends on another Orbitz customer booking the same exact itinerary. Likewise, Travelocity's price guarantee is less than ideal, since it requires you to find the lower competing rates yourself after you book your trip.)

In short, these three services together save you the time and hassle of hunting for the best prices for your next trip and possibly overpaying. Combine this with MySeatFinder's automatic upgrade to a better airline seat, and you've turned the web into a pretty sweet personal travel assistant.